Will a 67-72 A-body GM rear from a chevelle fit a 67 camaro?

No !

1) A - body's rear is wider.

2) A - body's are coil springs not Leaf Springs.

64-67 A body is the same width. 68-72 is 1" wider.You can weld the mounts on for leaf springs.So yes it will fit but its not a bolt in.

I'd install a 9" before I'd do that.

pdq67